So is it possible to have an autofocus motor as a separate attachment to the camera? This would go between the camera and the lens, like an extender. I think it should be doable in principle, but not sure if anyone makes them..

2 Answers
2

The problem with putting something between the lens and the camera is that you've moved the lens farther away from the sensor than it was designed to be. If your attachment doesn't have any optics in it, you've basically added an extension tube, and the lens will only be able to focus at close distances (possibly very close distances, like macro photography).

You might be able to design such an adapter that includes optical elements, basically a teleconverter (ideally, with 1x magnification, like a system of relay lenses). The problem is that any added optics have the potential to degrade the image, and there will be a cost (possibly substantial) associated with the optics. It is likely that there isn't much of a market for such a device—phototogrpahers willing to pay for it would probably just buy a body with an AF motor, or lenses with motors, instead.

On a tangent: When Pentax introduced AF cameras (which had built-in motors), they also released a 1.7x teleconverter that could provide autofocus capabilities with manual-focus lenses. The camera motor moved components of teleconverter, altering the focus of the lens. The range of autofocus was limited, so the MF lens had to be manually focused to approximately the right distance first.

You are comparing two lenses in different price ranges. Some of the price difference naturally comes from the focus motor, but most of it comes from the more expensive one having:

Vibration Reduction (VR)

More advanced optics (17 lens elements compared to 13)

Extra-low Dispersion glass (ED)

Internal Focusing (IF)

There are certainly cheaper lenses with that zoom range that has a focus motor, even if Nikon doesn't make them. You can get a Tamron AF 70-300/4,0-5,6 Di Macro for Nikon for about $150.

As for a motor attachment for Nikon cameras, that is not likely to appear. As the target group would be people with a newer type camera body and older type lenses, it's a shrinking market where there isn't any lasting profit.